In the handling of molten metal, particularly reactive metals such as zinc, great difficulty has been experienced in achieving low flow rates, with accuracy and without contamination.
Molten metals, particularly zinc, are difficult to handle, being extremely hot and chemically highly reactive, and attack stainless steels quite rapidly.
The most generally adopted prior art pumps have included pumps having a graphite rotor and surrounding casing, submerged vertically into a bath of molten metal. These pumps act centrifugally, to deliver the molten metal upwardly through an external vertical discharge pipe, under centrifugal pumping action. This arrangement requires that the rotor operates within a base plate housing, to discharge through the discharge pipe. The rotor drive shaft extends downwardly through the metal in the bath, being generally of graphite, and of significant diameter and in use creating considerable turbulence, so as to entrain dross and other impurities from the surface, some of which are drawn into and delivered with the pump output. This turbulence and entrainment is highly deleterious to an unprotected delivery pipe and other elements of the system. The adoption of a slender shaft of moly-tungsten alloy, in order to mitigate the shaft turbulating effect is so expensive as to double the cost of some pumps.
The rate of delivery is controlled by regulating the speed of the driving motor. In view of the non-linear pumping characteristic in relation to speed, accurate control and constancy of operation are difficult, if not impossible, to achieve.
Furthermore, the prevailing minimum pumping rates are about 80 pounds per minute, which is impossibly high for certain applications, while, also the nature of the pump design results in violent stirring of the molten metal, with consequent entrainment of dross into the delivered molten product.
The known prior art liquid metal pumps are prone to being readily disabled, by "freezing" of the metal within the pump to a solid, entrained mass, particularly in the discharge pipe, due to its exposed nature.